Harahara Shichau! Oya ni wa Naisho no Kozukuri Seikatsu: Asagari Shimai-hen

Haruki and Kotoko become lovers on a fateful afternoon in a shinto shrine dyed by the glow of the evening sun. All is going well until the announcement of their parents' marriage. The father of Haruki would marry the mother of Kotoko making them stepsister and stepbrother in an instant. Now they are going to live together not as lovers but as siblings. What will happen to their relationship.

(Source: VNDB)